Equant (or punctum aequans) is a mathematical concept developed by Claudius Ptolemy in the 2nd century AD to account for the observed motion of the planets. The equant is used to explain the observed speed change in different stages of the planetary orbit.

The equant is the point from which each body sweeps out equal angles along the deferent in equal times. The centre of the deferent is midway between the equant and Earth.

The "equant" is an imaginary mathematical point that "sees" the planet move at a uniform angular speed. Because it is not located in the center of the orbit it "causes" the planet's actual speed on the orbit to change.

2025年3月7日 · The equant is the point from which each body sweeps out equal angles along the deferent in equal times. The centre of the deferent is midway between the equant and Earth. (more)

2016年5月17日 · Equant was introduced by Ptolemy (100-170 ad) who built a satisfactory model for the whole Solar system. This model, with minor variations was used successfully until 17th century, when it was replaces by Kepler's laws, where the motion is on ellipses and the angular velocity is not constant.

Ptolemy made two great advances on Hipparchus, on was the deferent/epicycle model for the planetary motion, and the other was the equant model for main (deferent) cycle of the planets. Epicycle.

Ptolemy found that the angular rate at which the epicycle traveled was not constant unless he measured it from another point, which he called the equant. Midway between the equant and the Earth is the center of the deferent, called the eccentric.

2016年1月5日 · Ptolemy caused planets to slightly change their speed throughout their orbit, by using a mathematical point called the equant. This video explains how. Kep...

Equant (noun) - A point introduced in the geocentric model of the universe by the ancient Greek astronomer Ptolemy. The equant is used to account for the observed speed variations in the orbits of the planets.

In an epicycle model, a planet orbits on an epicycle which orbits on a deferent. The eccentric Earth and the equant are special features of the epicycle models of the Ptolemaic system.